Production and Isolation of Carrier-Free Radioisotopes

PRODUCTION AND ISOLATION O F CARRIER-FREE RADIOISOTOPES WARREN M. GARRISON

AND

JOSEPH G. HAMILTON

Crocker Laboratory, Radiation Laboratory, Divisions of Medical Physics, Experimental Medicine, and Radiology, University of California, Berkeley and S a n Francisco, California Received M a y 17, 1951 CONTENTS

1. Introduction.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 11. Sources of carrier-free radioisotopes.. . . ......... . . . . . . . . . . . . . . . . . 239 A. Pile reactions.. . . . . . . . . . . . . ................................. 239 1. Nuclear fission. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 2. Neutron capture followed by 8-emission.. . . . . . . . . . . . . 242 3. Neutron capture with proton and a-particle e B. Cyclotron reactions ..........

111. General considerations in preparation of carrier-free radioisotopes.. . . . . . . . . . . . 248 .............................. 248 A. Types of separation processes. . . . . 1. Coprecipitation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Leaching.. . . . . . . . . . . . . . . . . . . ........... 3. Radiocolloid formation. . . . . . ........... . . . . . . . . . . . . . . 253 .............

..........

I. INTRODUCTION

In the separation and use of radioactive substances it is frequently necessary t o isolate radioisotope preparations which do not contain detectable amounts

of stable isotopic material. These preparations, usually designated by the term “carrier-free,” are produced principally by nuclear transmutation reactiom2 This document is based on work performed under Contract No. 7405-eng-48 for the Atomic Energy Commission. This is Document No. UCRL-1067 based on work in unclassified areas of research under provisions of Bulletin GM-93, and its release has been recommended by the members of the Laboratory Review Committee as compatible with t h e stipulations of the Guide to UnclassifLed Fields or Topics of Research and the Declassification Guide. * Nontransmutation nuclear reactions may also be used t o produce carrier-free radioisotopes. A successful separation in cases where the target and product are isotopic depends on the removal of the radioactive atom by breaking its chemical bond in the target 237

238

WARREN M. GARRISON AND JOSEPH G. HAMILTON

Since transmutation products are generally obtained in amounts which are detectable only by their characteristic nuclear radiations, conventional inorganic separation procedures are usually not directly applicable and special methods must be employed. Many of these methods were originally developed in the investigation of the naturally radioactive elements. Even prior to the general acceptance of the theory of isotopy, a great deal of information had been obtained on the chemical behavior of unweighable amounts of the short-lived radioelements, including methods for their separation and isolation. Although few generalizations were then possible, the introduction of the disintegration theory, which was soon followed by the arrangement of the radioelements in the periodic table, rapidly resulted in a fairly comprehensive understanding of many of the unique chemical, physical, and biological properties of unweighable amounts of the radioelements found in the natural decay series. Probably, however, the most extensive developments in the chemistry of the radioelements in extreme dilution have followed the discovery of artificial radioactivity. The development of the cyclotron and the chain-reacting pile resulted in a tremendous increase in the number and availability of radioisotopes that can be produced in the carrier-free state. And with this, there has been a corresponding increase in our understanding of the fundamental principles of the separation processes. Many new techniques, particularly adapted to carrierfree radioisotope separation, have been developed. These techniques and those previously employed in classical radiochemistry have been successfully used in the separation of carrier-free radioisotopes of most of the stable elements and in the discovery and preliminary chemical identification of several new elements. The use of carrier-free techniques is essential in certain types of research and highly desirable in many others. It is, of course, fundamentally important to most investigations involving those radioelements, either artificially produced or naturally occurring, of which no stable isotopes have been found in nature. The fact that these substances are normally encountered only in invisible amounts necessarily delimits the type of chemical technique which can be employed in their manipulation. The isolation of carrier-free radioisotopes of the stable elements also is important in several widely differing fields of research. I n chemistry they have been used to investigate (a) the chemical properties of the elements a t extreme dilution, (b) adsorption phenomena, and (c) the properties of radiocolloids, in addition to ( d ) their use as tracers in many chemical problems in which high-dilution factors are important. Carrier-free radioisotope preparations are also desirable in certain types of physical measurement; for example, in the characterization of low-energy nuclear radiations, the use of a “mass-less” source is important to insure small self-absorption and in mass spectrographic determination of nuclear properties to obtain a maximum sensitivity. or parent compound as a result of isomeric transition (29, 117) or recoil (85, 123). Theae methods, although of considerable theoretical interest, are a t present not generally practical for the production of carrier-free tracers.

PRODUCTION OF CARRIER-FREE RADIOISOTOPES

239

The widest use of carrier-free activities and probably one of the most significant is their application to biological and medical research, since these preparations may be added to biological systems without changing the mass or concentration of the stable element or compound already present. This situation is extremely desirable, for example, in metabolic studies, in the investigation of the functions of trace elements, and in the use of radioisotopes as in vivo radiation sources. An important medical problem involving the metabolism of carrierfree radioisotopes has resulted from the quantity production of plutonium. The need for investigating the radiotoxicological hazards of the fission products and of the transuranium elements, neptunium and plutonium, has necessitated the biological testing of each of the radioisotopes in the carrier-free form in which it would be encountered in the plutonium process. Because of the importance of evaluating the health hazards of these materials, a great deal of information has been obtained on the metabolic properties of the radioisotopes produced in fission.

11. SOURCES OF CARRIER-FREE RADIOISOTOPES The chain-reacting pile and the cyclotron are the only practical sources of carrier-free radioisotopes, although a few useful radioisotopes are still obtained from the natural decay series. The important nuclear reactions for each of these modes of production are described briefly below. A . PILE REACTIONS

Neutrons at pile energies produce radioisotopes in the carrier-free state by four main types of nuclear transmutation reactions.

1. Nuclear fission I n the comprehensive investigation of the radioactive species formed in the fission of uranium more than 160 radioactive isotopes have been identified, fission from Z = 30 (zinc) to Z = 63 (europium), ranging in the case of P5 and from A = 72 to A = 158. As a result of the fact that the neutron excess of U235is considerably greater than that required for stability in the fissionproduct region, the primary fission-product nuclei achieve stability through successive O--decay, giving rise to fission-product chains. The radioisotopes in each chain, their relationships, their mass assignment and yields have been determined in most cases (103, 116). Although these radioisotopes are byproducts of normal pile operation, only a few have half-lives sufficiently long to warrant their routine separation. The major radioactive fission products of halflife greater than 1 week are described in table 1. Most of these are available in carrier-free form from the Isotopes Division, Atomic Energy Commission, Oak Ridge, Tennessee (64). 2. Neutron capture (n,y) followed by &emission

With the exception of the (n,f) reactions, the (n,?) reaction is the most important source of carrier-free radioisotopes in the pile. Although the ( n , y )

240

WARREN M. GARRISON AND JOSEPH 0. HAMILTON

reaction alone results in the production of radioisotopes which are isotopic with the target element and hence not separable in the carrier-free state, it may Major radioactivc

TABLE 1 ssion products of half-life than 1 week (6.4, 103, 116) . greater . EMPGY

PADIOISOTOPB

RELATIVE

HALF-LIPS

YEW*

Beta

Gamma

M.c.r.

M.c.v.

curies

ssSr*@. . . .... . ..... . . ..

55 days

1.48

None

1.0

&got. . . . . . . . . . . . . . . .

25 yr.

0.65

None

0.035

. . ... . . ....... ...

57 days

1.53

None

1.25

,oZr$'. . . . , . . . . . . . . . . . .

65 days

0.394 (98%)

0.73 (93%)

1.35

r ~ N b ~. .~. .. .

35 days

0.151

0.75

1.7

m y 91

4 3 T ~ 9 9 .,

. ............

.

,,RU'o= . . . . . . . . . . . . . . .

N

5 X 106 yr. 42 days 1.0 yr.

0.32 0.2 (95%) 0.04

None 0.56 None

0.7 X 0.9 0.065

90 days

I.T.,e-

0.086

0.0075

a2Te1*gm.. . . . . . . . . . . . . .

32 days

I .T.,e-

0.102

0.03

. . . . . . . . . . .. . . . . .

8 days

631"'.

,

., . ..... .......

s&il4Ot.

.... . . . . . .. . . .

&s"7

&e141.

.......

&e144t. . . . . . . . . . . . . .

*

.

,,Pm147,. . . . , . . . . . . . . .

33 yr.

0.35; 0.60 0.55; 0.84

0.63; 0.36; 0.28

0.065

0.66

0.032

12.5 days

1.05

0.529

0.35

28 days

0.66

0.2

1.0

275 days

0.35

None

0.85

13.8 days

0.83

None

0.4

11 days

0.90

3.7 yr.

0.22

0.55 None

10-6

0.12 0.14

* Relative amounts of fission products coexisting one month from the end of a severalmonth exposure (103). t Indicates that the radioisotope decays t o a more energetic daughter (103, 116). be used t o produce carrier-free radioisotopes if the product isotope decays to give a radioactive descendant. Table 2 describes the carrier-free radioisotopes which are routinely produced in the pile using this type of nuclear reaction. The activation cross-section of

241


TABLE 2 Currier-free radioisotopes produced in the pile by (n,r)reaction followed by decay (64,116,119)

saAs'7. . . . . . . . . . . . .

40 hr.

4iNbss.. . . . . . . . . . .

35 days

43Tc97". . . . . . . . . . . .

93 days

...........

4zT~99.

N

ISOTOPE

HALF-LIFE OF

PEF. CENT LBUNDANCB

n, 7 ) PRODUCT Isotope

Natural

6.5

0.085

0.0055

12 hr.

17.4

0.06

0.014

65 days

2.2

0.15

5.68

2.8 days

5 X 106 yr.

24.1

0.415

0.10

37 hr.

18.27

0.667

0.122

4.4 hr.

13.5

0.39

0.053

26 months

6.8

0.574

0.039

9 months

34.52

0.22

0.0735

25 months

46Rh'06 . . . . . . . . . . . 47Ag'l'.

IABGET

HALF-LIFE

PADIOISOTOPE

...........

7.5 days

61Sb'*'. . . . . . . . . . . .

2.7 yr.

67 hr.

.............

8.0 days

...........

10.2 days

aPrl4*.. . . . . . . . . . .

13.8 days

11.7

0.95

0.11

33 hr.

,1Pm147...........

3.7 yr.

17.1

1.4

0.24

11.0 days

1.7 hr.

631131,

ssCsl3'.

12.0 days

0.101

slPm14g. ..........

47 hr.

5.78

2.8

0.16

..........

2 yr.

22.53

5.50

1.10

25 months

..........

3.3 days

7.2

3.92

0.28

31 months

0.015

0.015

5.0 days

&U166. 79

Au' g 9 .

uPo21Q.. . . . . . . . . . .

100

140 days

Carrier-free radioisotopes produced in the pile by (n,p) and ( n p ) reaction WIOISOIOPS

I

EALe-LIPB

rH' . . . . . . . . . . . . . . . . . . .

12 yr.

BC14. . . . . . . . . . . . . . . . . . .

5100 yr.

PSPEPENCLS

PPODUCED BY

(97) (96, 108, 134)

1sP32. . . . . . . . . . . . . . . . . .

14.3 days

(16, 30, 32, 47, 114)

.................

87.1 days

(68, 110, 114, 131)

l W 6 . &a46.

................

180 days

(9, 72)

the target isotope and the characteristics of the radioactive descendant are also listed.

242


3. Neutron capture with proton and a-particle emission: (n,p ) (n,a ) reaction

A few important ( n ,p ) and (n,a ) transmutation reactions are possible with pile neutrons. These are confined to the production of radioisotopes of the lighter elements, because only a negligible fraction of pile neutrons have energies sufficient to overcome the potential barrier to charged-particle emission above Z = 20. Resonance in certain nuclei also permits (n , p ) and (n ,a ) reactions to proceed with neutrons of low energy. Table 2 lists the radioisotopes which are produced by (n , p ) and (n ,a) reaction in the pile. B. CYCLOTRON REACTIONS

The cyclotron is the only practical source of a large number of useful carrierfree radioisotopes which are not available from pile sources (64, 116). In addition, although a desired radioisotope may be produced in the pile, half-life and other technical limitations may make cyclotron irradiation the more desirable method of production. With the exception of the (n,f ) reaction, charged-particle bombardments differ from pile reactions in the greater probability of concurrent reactions. This is particularly true at the higher bombardment energies. With 20-M.e.v. deuterons, for example, three and occasionally four radioactive elements may be produced concurrently from the target element (130). At energies considerably higher than this (-100 M.e.v.) a whole spectrum of transmutation products is observed. Reaction products covering a range from the region of the target nucleus down to nuclei about 20 mass units lighter are produced by bombardment with particles having energies in the hundred-million-volt range (25, 46, 86). To indicate these reactions, in which excited nuclei are degraded by losing one or more nucleons, the term “spallation” has been suggested. The majority of the artificially produced tracers are produced by the relatively simple reactions involving the emission of not more than three or four nuclear particles. The intensity of the charged particles is low a t these energies and the radiochemical procedures become formidable, owing to the large number of radioelements produced. Because of the great variety of nuclear transmutation reactions (115, 116) which can be brought about by bombardment with deuterons, protons, and alpha particles, it is frequently possible to produce a desired radioisotope by several different types of nuclear reactions. In determining the particular reaction to be employed, several factors must be considered, including (1) the yield of the desired radioisotope, (2) the relative yields of the possible concurrent side reactions, (3) the problem of the ultimate chemical separations of the desired product, and (4) the chemical and physical limitations which must necessarily be imposed in the choice of target material for charged-particle bombardment. Considerations 1 and 2 are discussed below and 3 and 4 are covered in the section on separation procedures.

PRODUCTION O F CARRIER-FREE RADIOISOTOPES

243

1. Deuteron reactions

For the production of many useful radioisotopes in the carrier-free state the reaction types @,a), ( d , 2 p ) , ( d , n ) , and ( d , 2 n ) are the most important. The ( d , a ) and ( d , 2 p ) reactions are most useful in producing radioisotopes of lower atomic number because of the potential barrier to charged-particle emission which increases with atomic number. Although the yield of these reactions increases with deuteron energy, at the higher energies the ( d , n ) and ( d , 2 n ) reactions become relatively more important for the elements of higher atomic number. The ( d , a p ) reaction is relatively unimportant in the production of practical amounts of radioisotopes, although the possibility of this reaction must be considered as a source of radioactive contamination. I n practice, the above transmutation reactions and the nontransmutative ( d , p ) reaction all occur concurrently, with reaction probabilities determined by the energy of the bombarding deuteron and by the atomic number of the target nucleus. The reaction probability (u) of competing reaction types and their energy dependence (thin-target yields) have been determined for a number of target elements, using the stacked-foil technique (14). Nuclear reactions for which excitation functions (u versus E ) have been determined are listed with references in table 3. I n radioisotope production by charged-particle bombardment the target is generally thick enough to absorb the entire beam. Thicktarget yields may be obtained by graphical integration of the particular excitation function involved (14), although usually they are obtained directly from thick-target bombardments. Available thick-target yields for charged-particle reactions are given in table 4. 2. Alpha-particle (helium-ion) reactions

Alpha-particle capture followed by neutron emission-(a ,n)(a ,2n)(CY ,372) reaction-is useful in the production of certain radioisotopes, particularly of the heavier elements. The principal advantage of this type of reaction is the double increment in atomic number. The (a,zn) reaction is important, for example, in the preparation of radioisotopes of astatine (2 = 85), because the only target element available for this reaction is bismuth (2 = 83). The reactions of the type ( a , p ) and ( a , p n ) are more probable with elements having low atomic numbers, because of the potential barrier to charged-particle emission, Excitation functions for alpha-particle reactions are included in table 3. Thick-target yields are given in table 4. 3. Proton reactions All of the transmutation reactions induced by proton bombardment can be duplicated, as far as carrier-free radioisotope production is concerned, by deuteron bombardment. Deuterons are usually favored over protons because higher energies are more readily obtained. Yield data for a few proton reactions are given in tables 3 and 4.

TABLE 3 Excitation junctions of transmutation reactions Table of references TARGET

(NATURAL ELEXENT)

TRANSMUZATION P U C T l O N S

RE?ERENCE

yAXIMU16

ENERGY

Y.c.P.

c. . . . . . . . . . . . . . . . . N . ... . , . .. . . . . . . .

C (d ,n)N1'

5

N (n,p) 0 4 ; N (n,a)B"

1.7

0. . . . . . . . . . . . I . . . .

0 (P ,n)

4

Mg . . . . . . , . , . . . , . . .

Mg(d, a)NaZ4 Mg ( d , a)NaZ*-24

3.5 14

Al(d, ap)NaZ4

14

,

AI . . . .

.. . ... . . . . . .

s. . .......... . . ...

S(n,p)P'*

c1. . . . . . . , . . , , . . . ,

c1(n,p) S'6

6.8

Cl(d, a)Sas

14 14

Fe . . . . . . . . . . . . . . , .

Fe (d,n)Co66 Fe(d, n)Co66; Fe(d,a)MnS*

10 10

Ni . . . . . . . . . . . . . . . .

Ni (p ,n)Cu613 Ni(p,n)Cu64 Ni ( d , n)Cu61

cu... . . . .

ZII . .

I

,

,

. .... . .

..... . . . . .. . .

Se . . . . . .

... .

Cu(p, n)Zn6' Cu(d ,2n)Zn6' Cu(p, n)Zn68 Cu(d,2n)Zn6'~66; Cu(d, a)Ni"

6.3 4 5

6.3 16 4 14

Zn(p,n)Ga64f6a,ro Zn(p,n)Ga68v'0

4

Se (p,n)Brso.a2 Se(p,n)Br*o,*S

6.3 4.0

6.3

Br . . . . . . . . . . . . . . . .

Br(d,2n)KP

13.5

Rh . . . . . . . . . . . . . . .

Rh(a, n)Aglo6; Rh(a ,2n)Ag'OL

19

Pd . . . . . . , . . . . . . .. .

Pd(d,n)Ag'O'; Pd(d,2n)AglO6

6

Ag . . . . . . . . . . . . . . . .

Ag(d,2n)Cd'07,'Og Ag(a, n)In1I2;Ag(a, 2n)In"' Ag(cy,zn)In1Og."0,"'

9 19 37

In . . . . . . . . . . . . . .

In(cy,n)Sbl1S; In(a,2n)Sb117; I n ( a , 3n) Sbl16

38

... . . .

Au(d,2n)Hg1g7

9

TI . . . . . . . . . . . . . . . .

Tl(d,n)Pbzo6

9

Pb. . . . . . . . . . . . . . . .

Pb ( d ,n)BiZo7

9

Bi . . . . . , . , . , , . . . . .

Bi(d,n)Po2'0 Bi(d,n)Po210 Bi (d ,n) Po21o; Bi ( d , 3n)Pozoa Bi(d,n)PoZ10 Bi (a, 2n)AtZ1l; Bi (a,3n)At2I0

9

Au . . . . . . . . .

,

244

9 19 14.5 38

245


TABLE 4 Thick-target yields of some artificial radioelements (66) Yields are given in terms of microcuries per microampere hour. The term "microcurie" refers to the absolute number of disintegrations per second of the artificially prepared radioelement: 3.7 X lo4 disintegrations per second. This definition] however, does not apply to those radioelements, with the exception of beryllium, whose yield values are enclosed in parentheses, since they are all substances decaying by orbital electron capture and the measured radiation contains varying proportions of x-rays, gamma rays, and internally converted electrons. I n such instances, the value of the microcurie is a comparable one and simply indicates that the amount of ionization produced is equivalent with the measuring device employed to that from 1 microcurie of the radioactive standard. A Lauritsen electroscope with a thin-walled aluminum window was employed for these measurements. The total air equivalent of the window, together with the distance of the sample from the instrument, totalled 4 cm. The instrument was calibrated by means of UX1 standards which were covered with a sufficient thickness of aluminum foil to screen out most of the soft UXI betas and approximately 15 per cent of the more energetic UX2 beta particles. The values presented in the table include yields for deuterons a t energies of 8, 14, and 19 M.e.v., and helium ions a t 38 M.e.v. The last column gives the factor of the differences of isotopic abundances of the target materials. This correction was not applied t o the yield data. In the case of C14, the value given is a calculated one assuming a bombardment of 100 gallons of saturated solution of ammonium nitrate. The accuracy of the underlined yield values is believed to be accurate t o f 2 5 per cent. Those not underlined are subject to much greater error. __

YIELD IN MICROCURIES P E E MICROAMPEBE EOUX ISOISOTOPE

HALF-LIFE

EADIATIONS

XEACTION

8-

hi.e.v. deuterons

12.4 yr,

B-

52.9 days

K,

21 months

B+

5100 yr.

B

10 months

8+

112 months

rst

2 . 6 yr.

B+,

Y

14.9 hr.

8-,

Y

10.2 monthf P-,

Y

10.2 monthr 8-,

Y

-

14.1 days

8-

87 days

8-

ropz

14-M.e.v. deuterons

19-M.e.v. deuterons

18-M.e.v.

0.1

1 2-

Y

1.1

500

5 0.0005

1

1000

1

-

OOO

1.0

800

1 9

2500 30

1 9

1500 500

%TI0

-particles

120 -

1 1

8.4

4

-

* Calculated from known neutron yields and pa for nitrogen in 100 gallons of a saturated solution of ammonium nitrate.

246

M. GARRISON AND

WARREN

JOSEPH Q. HAMILTON

-

TABLE 4-Continued YIELD IN MICPOCOPIES PEB HALF-LIFE

PADUIIONS

PEACIION

8-

M.e.v. deu-

.erons

MICROAMPERE HOUR LSO-

'OP?.

lCM.e.v.

deuterons

19-M-e.v. ieuterons

8-M.e.v.

AX10

particle!

-

-

1000

4

38 months

8-,

Y

12.4 hr.

e-,

Y

12.4 hr.

8-7 Y

12.4 hr.

8-9 Y

300

1

22.4 hr.

8-,

d00

1

152 days

6

85 days

B-,

26 days

K , e-,

6.5 days 310 days 46 days

4 yr. 270 days

50

107 -

50

3.3

Y

0.01

-

0.05 0.01

Y 7

K , P, Y Y

(0.10)

8 ,Y

0.03 -

K,

15

K , B+, e-,

(1.0)

1.3

(0.2)

1

(81

1.2

1.1 60 (0.02

K

50

(5.0)

17 1.1

Y

12.9 hr.

K , 8-, B+,

1.5

3000)

Y

12.9 hr.

(300)

K , B, B+,

2.1

Y

60 hr.

250 days

10 -

8K , 8+, e-,

25 3

(0.5)

Y

78 hr.

K,

e-, Y

(30)

3.8

11.4 days

K,

e-, Y

(8)

2.5

17.5 days

B+, 8-2

Y

2

10 -

3

-

247


TABLE &Continued

HALF-LIFE

RADIATIONS

8-

Ke.v. deuterons

YIELD IN YICPOCUBIES PEP Y I C P O A W E P E HOUP

14-M.e.v.

deuterons

19-M.e.v.

deuterons

38-M.e.v

-particle

-

127 days

K , e-,

35 hr.

8-, r

19.5 days

B-,

Y

65 days

K,

Y

54 days

8-

105 days

1

(1 1

7

500 1.0 (0.13) -

7

(0.10) 7.0 -

10 1.2

(0.60: -

1.2

10.4 -

K , B+,

ISOrOPE LATlO

1.2

(1.0)

1.2

75.0

1

78 hr.

8+

65 days

8-, e-,

16 hr.

8+, Y

55 days

K , e-,

Y

(1.0)

2

37 days

8-, e-,

Y

0.05 -

11

67 hr.

B-,

Y

0.15 -

5.9

2.4 -

6.2

0.1

Y

2.8 days

K , e-,

7

4.3 days

K , e-,

Y

(30)

@)

36

t 7.9

17 days

K

(0.05)

1

40 days

K , e-, r

(0.10) -

7.4

K

(2)

2.1

470 days 2.8 days

K , e-,

Y

(0.10:

50 days

I.T., e-

t Di5cult to evaluate owing t o multiple reactions or more than one target isotope.

t -

248


TABLE 4-Concluded

YIELD IN MICROCUPIBS PEE MICROAMPERE EOUR

SOHALF-LIP1

IADIATIONS

REACTION

8-

Il.e.v. deu:erons

105 days

K , e-,

Y

6.0 days

K , e-,

Y

2.8 days

8-, e-,

Y

143 days 8.0 days 38.8 hr.

19-M.c.v.

deuterons

R-M.e.v particle

OPE AT10

(0.50 -

t

1

t

e-, Y

8 ,e-,

14-M.e.v. deuterons

Y

1.(

I.T., e-, y

(0.16)

1.8

20.0 -

t t

100)

t

140 days

K , e-,

Y

240 days

K , e-,

Y

52 days

K , e-,

Y

10.7 days

K , e-,

Y

70 days 52 hr.

K , e-, K , e-,

Y Y

3.4

K , e-,

Y

3.9

6.4 days

1

t

(0.05

t

1

2.0

138 days

Ql

Y

138 days

a1

7

0.85

2

a, K

:100)$

1

7 . 5 hr.

-

~

$ Yield for 29-M.e.v. helium ions.

111. GENERALCONSIDERATIONS IN PREPARATION OF CARRIER-FREE RADIOISOTOPES A. TYPES O F SEPARATION PROCESSES

Although the detailed methods which have been developed for isolating carrier-free radioelements are extremely varied and depend on the particular


249

problem at hand, all are based on a relatively few general types of separation processes: (a) coprecipitation, (b) leaching, (c) radiocolloid formation, ( d ) electrodeposition, ( e ) ion-exchange, (f) solvent extraction, (9) distillation (volatilization). Each of these is discussed briefly below in terms of its applicability to the problem of isolating univeighable amounts of the carrier-free radioelement, or compound of the element, in aqueous solution a t a pH range of 3 to 8. This criterion has been adopted for two reasons: First, because the most extensive use of carrier-free radioisotope preparation is in biological and medical usage, for which it is usually desirable to obtain radioactivities of extremely high chemical, radiochemical, and isotopic purity in isotonic solutions of sodium chloride, sodium sulfate, or other nontoxic salts. Second, because these specifications are generally high enough to meet the requirements of almost all other types of investigation requiring carrier-free radioisotopes. 1. Coprecipitation

The separation of submicrogram quantities of radioelements from solution by means cjf nonisotopic preciFitates usually involves a subsequent separation of the radioelement from the carrier precipitate. It is deEirable, therefore, to use, as coprecipitating agents, substances which can be easily separated with a minimum amount of chemical and physical manipulation. True coprecipitation processes (51, 61) (i-e.,those cases in which the radioelement because of similar chemical properties is incorporated in the precipitate through isomorphism or mixed crystal formation), although to a large extent extremely reproducible and independent of precipitating conditions, are not of great practical importance, particularly in the isolation of radioisotopes for biological research. The inherent difficulties involved in the subsequent separation of the chemically similar radioelement and precipitant frequently more than off set the desirable features of coprecipitation processes. This factor has, however, become somewhat less limiting with the development of the ion-exchange technique (127). More generally in the separation of carrier-free activities a “scavenging” type of precipitation reaction is employed. In these cases, the radioelement is carried down as a result of adsorption phenomena. Although precipitation reactions of this type are quite sensitive to changes in experimental conditions and are not as specific as the true coprecipitation reaction, they have the important advantage that the scavenging precipitate can usually be chosen so that its subsequent separation from the carrier-free radioisotope involves a minimum amount of manipulation. Table 5 is a r6sum6 of some important coprecipitationscavenging types of precipitation reactions. A serious disadvantage of coprecipitation processes in general is the fact that they involve the addition of macro amounts of carrier material which may contain impurities which are isotopic with the desired radioisotope. In addition, coprecipitation reactions, particularly those involving adsorption phenomena, are frequently specific only under rigidly controlled experimental conditions (61). 2. Leaching The extraction of carrier-free radioisotopes into aqueous solution from macro quantities of insoluble target or parent material has been used as a separation

250

WARREN M. GARRISON AND JOSEPH Q. HAMILTON

TABLE 5 Classification of separation processes used i n the isolation of carrier-free radioisotopes References t o the detailed methods outlined in table 6 are given in parentheses SEPARATION PROCESSES WHICH HAVE B E E N EMPLOYED

PRODUCED BY

XADIOISOIOPE

Coprecipi- Solvent extraction tation

Ion exchange

>as evoluon vapor

Radb

adon, an( :olloid listillation

-

Ha. . . . . . . . . . . . Be'. . . . . . . . . . .

Leaching

Electrodepsition

(66, 97) (2, 24,

(54)

65)

C" . . . . . . . . . . .

(12, 109)

C" . . . . . . . . , . .

(96, 108 131)

N". . . . . . . . . . .

(95, 105)

F1*. . . . . . . . . . .

Na*'** 4 . . . . . . .

. .. . . . . ..

Mg'7.

(53)

Pa*. . . . . . . . . . .

(47)

S". . . . . . . . . . . A". . . . , . . . . . ,

IC",

48.

. . . ... .

Ca46. . . . . . . . . .

SC46. V'8.

(52)

(9)

. .. . . . . . .

. . . ......

.

Cr61.

.... . ....

Mn6'.

s4. . . . . .

(89)

25 1


TABLE 5-Continued SEPARATION PROCESSES WHICH HAVE BEEN EMPLOYED EADIOISOTOPE

Mn" . 64 . . . . . . Fe'9

PRODUCED BY

zoprecipitation

Solvent extraction

Ion exchange

.

.

ladio:olloid

2ectroieposition

Leaching

.

..........

C066.

67

.

CU649

67

.......

b8.

... (31)

Zn66 . . . . . . . . . . Gs67. . . . . . . . . .

Gei1. . . . . . . . . . Asi4 . . . . . . . . . .

Se76. . . . . . . . . . Br80s ** . . . . . . .

Kri9 . . . . . . . . . . Sr*6. . . . . . . . . .

SrB9. 9'3 . . . . . . .

YO*. . . . . . . . . . . Y 01 . . . . . . . . . . . Zrso . . . . . . . . . . Zr $ 5 . . . . . . . . . . Cbg6 . . . . . . . . . .

MoB9. . . . . . . . . Tc96. . . . . . . . . . Tcgg. . . . . . . . . . Rul"

.........

.

.

252


TABLE &Continued SEPARATION PROCESSES WHICH HAVE BEEN EMPLOYED

-

PADIOISOTOPE

PBODUCED BY

Solvent extraction

Radioc oI I oid

-

Leaching

Electrodeposition

Pd103. . . . . . . . . . Rh(d,2n) AglOS, 10(. Aglll.

111..

.

. ........

Pd(d,2n) Pd111 @-decay

(49)

Cd"J9. . . . . . . . . . Ag(d,2n) In1119 114.

. .....

Cd(a,pn)

Sn113. . . . . . , . . . , C d ( a , m ) Sb120*12*.. . . . . . Sn(d,sn) Te1278 129.. , . . , , 1181.

,

.. . ... ...

u(n ,f)

.I Te(d,sn)

P1. . . . . . . . . . . , U(n,f) Ilal., . . . . . . . . , . Te131 ,+decay

Balas.

. . . . .. . . .

Ca(d,2n)

Bal4O. . , . . . . . . . U(n,fl La140. . . . . . . . . . Ba140 8-deca y

(98)

Rare earths: Z = 58 t o 2 = 63... . . . U(n,f)

I___

---

253


TABLE &-Concluded SEPARATION PBOCESSES WHICH EAVE B E E N EPIPMYED RADIOISOTOPE

Rare earths: 2 = 64 to 2 = 71 . . . . .

PBODUCED BY

CoprecipiSolvent tation Extraction

Z(p,zn) (d ,xn) (a zn)

(132)

Hf (d ,zn)

(56)

3

Tal??,178,

180, ,

Re’s3, l S 4 . .. . . .

Ta(a,xn)

. .. . . . . . .

W(a,zn)

Bi204f2 0 8 , . . . . .

Pb ( d , 2n)

P0210. . . . . . . . .

Pb(a,zn)

(93)

........

Bi(d ,n)

(93)

AtZ10, ‘ll.. . . . .

Bi (a,z n )

0

~

5

P0210.

Ion exchange

?as evoluion, vapor iadiocation, an( :olloid iistillation

I

Leaching

Electrodepoaition

(4~43) (42)

(35, 67)

procedure in a few cases. These are described in table 5 . Although quantitative separations are rarely obtained, the relative simplicity of the leaching technique makes it a useful practical procedure. Since the carrier-free radioelement must exist, under the particular experimental conditions, as a soluble compound and must not be preferentially adsorbed or incorporated in the insoluble material, the separation has had only a limited application. 3. Radiocolloid formation

Carrier-free radioelements, under conditions which normally result in the formation of visible precipitates if a sufficient quantity of the material is present, may form radiocolloidal aggregates even though the solubility product conditions are not satisfied. Although the exact nature of this phenomenon is not entirely clear, it has been suggested (51, 78) that the radioelement becomes adsorbed on colloidal impurities which are normally present in the solution. Adsorption may also occur on the walls of the containing vessel. This phenomenon has been satisfactorily employed in the isolation of carrier-free radioisotopes of several elements, particularly those forming sparingly soluble hydroxides; an alkaline “solution” of the carrier-free activity is passed through filter paper or sintered glass, which removes the radiocolloidal material by preferential adsorption. The invisible quantity of adsorbed radioelement is washed with water and then removed with dilute acid. Radioisotopes which have been sepa-

254


rated by use of this technique are shown in table 5. The procedure is particularly useful in the isolation of radioactive tracers for biological investigation, since an isotonic saline solution may be obtained simply by neutralizing the hydrochloric acid wash.

4. Electrodeposition Radioelements which have been isolated in the carrier-free state by methods involving electrodeposition reactions include (1) electronegative elements which are reduced to the metallic state by displacement with a more electropositive element or by an applied electromotive force, (8) elements which form insoluble oxides by anodic oxidation reactions, and (5) elements which form insoluble compounds with the electrode material as a result of either cathodic or anodic reactions. Carrier-free radioisotopes which have been separated by electrodeposition reactions are given in table 5. Separation of the activity from the electrode material, particularly when platinum or other noble metals are used, is usually accomplished by preferential dissolution of the radioelement. With a mercury cathode, the separation is conveniently accomplished by volatilization. I n certain cases, however, the deposited activity can be removed only by treatment with chemical reagents which react with the electrode material, necessitating a subsequent separation to obtain a chemically pure solution of the activity suitable for use in biological systems. 6. Ion exchange The separation of trace amounts of radioelements by selective elution from ion-exchange columns has become an increasingly important process in the preparation of carrier-free radioisotopes, particularly in the case of the cationic fission-product elements. Although ion-exchange methods had been previously used, the separations obtained were not sufficiently good to warrant the use of exchange adsorption phenomena in radioisotope separations until the important effect of complex-forming agents on the adsorption-elution cycle was recognized. The necessity of isolating pure fission products for biological testing resulted in the development of this entirely new technique of ion-exchange separation, which has had its most important application in the separation of carrier-free radioisotopes of the rare earth elements from fission-product mixtures (127). I n this work it was found that a mixture of carrier-free fission-product activities adsorbed onto Amberlite or Dowex 50 ion-exchange resin could be selectively eluted with a dilute aqueous solution of organic complex-forming acids buffered to a controlled pH with ammonium hydroxide. In a typical separation, a dilute hydrochloric acid solution of carrier-free fission products is passed through a column of Amberlite IR-1 or Dowex 50 ion-exchange resin, which adsorbs the activities in a narrow band a t the top of the column. After thorough washing with water, the carrier-free radioisotopes are selectively eluted with 5 per cent citric acid solution, a t a pH of from 3 to 8, depending upon the type of resin, the rate of removal, and the separation desired. Under optimum conditions, the method is sufficiently precise so that a quantitative separation of neigh-


255

boring rare earth elements may be obtained. To obtain the carrier-free radioisotopes in hydrochloric acid solution, the citric acid effluent is acidified to reduce the complex-forming action of the citrate ion and passed through a second column, which readsorbs the activity. After washing with dilute hydrochloric acid, the activity is stripped from the column with 6 N hydrochloric acid. The ion-exchange technique is the only practical method available for separating many of the fission-produced radioisotopes in high purity and in a form which may be used in biological investigation with a minimum amount of chemical treatment and manipulation. The important ion-exchange separations are summarized in table 5 . 6. Solvent extraction

The selective extraction of a radioactive tracer as a nonpolar compound or complex from an aqueous solution by an immiscible organic solvent is frequently the most satisfactory method of separating the activity from nonisotopic substances which may be present in either macro or micro concentrations. These processes can often be made highly selective. Solvent extraction is also used to remove macro quantities of nonisotopic substances from an aqueous solution of a desired carrier-free radioelement. This method of removing a macro constituent from solution is of great importance when separation by precipitation would result in a substantial loss of the carrier-free radioelement by coprecipitation. As a rule, the distribution ratio of an extractable substance is more or less independent of the initial concentration, but the fact that a favorable distribution ratio is obtained a t macro or even micro concentration levels does not necessarily mean that submicro amounts of the substance will be extractable. For example, carrier-free radioiron cannot be extracted into ethyl ether from 6 N hydrochloric acid, although, as is well known, the distribution ratio of ferric chloride a t higher concentrations is sufficiently large to permit the use of ether extraction as a quantitative separation process. This concentration effect has been observed in other cases also and is generally assumed to indicate that the molecular weight of the partitioning substance is greater in the organic solvent as a result of polymerization. Usually, however, carrier-free radioelements at concentrations as low as M show extraction coefficients which are of the same order of magnitude as those obtained at the macro level. A number of organic substances have been used to form extractable nonpolar complexes or chelates with carrier-free radioelements. Compounds which form soluble nonpolar complexes whose dissociation constants are pH dependent are particularly useful in separating a carrier-free radioelement from a mixture. Extraction separations are summarized in table 5 .

7. Distillation (volatilization) Carrier-free radioelements which have been separated from solution, melts, and solids by volatilization or distillation processes are shown in table 5 . It has been found that the experimental conditions for the volatilization of a carrier-

256


free radioelement from solutions and melts, i.e., from a homogeneous liquid phase, are roughly equivalent to those found to be optimum for macro quantities of that element, although an inert carrier-gas is generally required. In volatilization separation from solids, the relative behavior of submacrogram amounts of a radioelement is to a great extent dependent on the nature of the solid and its physical state. In some cases, carrier-free radioisotopes of the inert gases produced by transmutation reactions in solids can be volatilized only after the solid target or parent material has been fused or dissolved in aqueous solution (52). Under proper experimental conditions, very sharp separations can often be obtained by volatilization methods. These procedures are more generally applicable, however, to the separation of carrier-free activities from solutions and melts than from solids. An important advantage, particularly in the isolation of radioisotopes for biological research, is that the activity can usually be condensed in a small volume of water or aqueous solution which can be used with a minimum amount of additional chemical treatment. B . PURITY CONSIDERATIONS

I. Target materials and reagents I n preparing carrier-free radioisotopes of the stable elements, care must be taken to insure that minimum amounts of isotopic impurity are present in the original target material and in the reagents used in the chemical separations. The precautions and techniques which are used in conventional trace analysis (111) to prevent chemical contamination are directly applicable to the problem cf preventing stable isotopic contamination in the preparation of carrier-free activities. A useful method of estimating the amount of isotopic impurity contributed by the target material, reagents, and vessels is to run a “blank” on the entire separation process. Spectrographic analysis of the final product will indicate the extent of the isotopic contamination. The availability and sources of elements and compounds in forms of high purity have been discussed elsewhere (23). Although stable isotopes as well as radioactive isotopes of the desired element may be produced in the transmutation reaction, the amount of element so produced is generally small in comparison with the amount of stable isotopic material which may be added unknowingly in the separation process. This may be the case even though the isotopic impurity cannot be detected by use of the most sensitive analytical procedures. The only radioisotope preparations which can be obtained absolutely free from admixture with stable isotopic material are of course those of elements which do not occur naturally in weighable amounts. Although there is a certain ambiguity in the term “carrier-free” in describing radioisotope preparations of the stable elements, it is generally used, as it is in the present paper, to indicate that stable isotopic carrier materials were not added to facilitate the separation and that “reasonable” precautions were taken to insure that chemicals of maximum purity were used.


257

TABLE 6 Outline of methods for the production and isolation of carrier-free radioisotopes RADIOISOTOPE

DESCPIPTION OF SEPARATION PPOCBSSES

RODUCED B1

IHS. . . . . , , . . . . .

The H a is releascd by heating the Be in a stream of oxygen or LiF in an evacuated quartz tube. The H3 is condensed as H:O after passage over hot CuO (66, 97).

,Bel..

Be7 is separated from Li by coprecipitation on Fe(OH)3from dilute NH4OH solution. The Fe(OH)$ containing the Be7 is dissolved in 6 N HCI and Fe is extracted with ether (24, 65). The Li is dissolved in water. The alkaline "solution" is drawn through a sintered-glass filter which removes the Be7 as adsorbed radiocolloid. The activity is removed from the glass surface with dilute acid (54). Be7 is extracted from aqueous solution at pH 5-6 by thenoyl trifluoroacetone in benzcne. The activity is back-extracted with concentrated HCI (2). ~~~

~~

SC"

. ..... ,....

C110 and C1lOz are expelled from a B20atarget during bombardment. The C"0 and C1lOz are passed over hot CuO, and the C1102 is condensed in a liquid-air trap (12, 109).

&14.

. .. .......

The C140 and CL4O2formed during bombardment of an NH4N03 target solution are removed by aspiration with C02-free air. The gas is then passed over hot CuO, and the C1402is collected in Ba(OH)nsolution (96, lOS, 134). Ca(NO8)Z target material is dissolved in a vessel filled with Con-free air. H20z is added t o oxidize nitrogen oxides to nitrates. HNOBis added and the Cl4O2is trapped in Ba(0H)Z solution (110).

. .......

The C is bombarded in a gas-tight chamber. The IV3released during bombardment is pumped off. The major part of the NIa remains in the C target material. This fraction is obtained by burning the C target in a combustion tube (95,105).

OF'S

. . . . . . . . . ..

Liquid H 2 0 may be bombarded in an adequately cooled thinwindow target to produce an aqueous solution of F1*directly (101, 128).

llNaZ2~ 24. . . . . .

The NanZt24 is dissolved out of MgO, Mg(OH)z, or MgCOt almost quantitatively with water (27, 48, 118).

,"3..

*

NanZv does not coprecipitate with M ~ ( N H ~ ) Z ( C O ~ ) ~ . ~ H ~ O (63).

A separation of Na*2 from weighable amounts of Mg may be obtained by the use of ion-exchange techniques (5).

258

W A R R E N M. GARRISON AND JOSEPH G . HAMILTON

TABLE 6-Continued RADIOISOTOPE

BODUCED B

izMg*7. . . . . . . . .

DESCRIPTION OF SEPARATION PROCESSES

The A1 target is dissolved in excess NaOH. The resultant "solution" is drawn through filter paper, which separates the Mg27 as adsorbed radiocolloid. The activity is removed with dilute HC1 (53). Pile-bombarded S (elemental) is melted, and a t a temperature of 120-13OoC. is poured into boiling concentrated HN03 with stirring. On cooling, the HNOs containing the Pseis separated from the S, and filtered. F e w is added and the solution is made alkaline with NHIOH. The Fe(OH), containing the Pa* is dissolved in HCl and Fe is extracted into ether. The Ps* solution is then passed through a cation-exchange column (16, 114). CSZmay be used as target material with other neutron sources (i.e., radium-beryllium or cyclotron). The Pa is extracted from CSz by shaking with dilute HNOs (30, 32).

Pa2is removed from irradiated CSZ by the use of electrodes immersed in the liquid (47). 1

'S". . . . . . . . . . .

KCl, NaCl, and FeC13are used as target materials. Separation of Ss6from irradiated KC1 or NaCl is effected by taking advantage of the insolubility of these salts in concentrated HC1. Ss6is quantitatively retained in the HCl solution. Separation from FeCla is effected by solvent extraction of FeC13 from HC1 solutions (68, 114, 131). KC1 is dissolved in H20 and a few drops of HzOZ are added to oxidize S36 to Soy-. K+ is removed on an ion-exchange column (110).

uA'7..

nK"*

&a".

. .... ... ~

'8

..... .

. . .. . . . .

Cl(d, 2n)

The Aa7is retained in the KC1 target material during bombardment. Separation is made by fusing the KC1 or by boiling a solution of the KC1 i n vucuo (52).

A stream of A is bombarded in a bell-jar target. The major portion of KQ, 43 settles out on the walls of the bell-jar and the remainder is caught in a glass-wool plug in the gas outlet. After bombardment, the walls and the plug are washed in warm HzO (99). The SCzOa target is dissolved in HC1, and reprecipitated as 3c(OH)s with the addition of NH40H. The Ca46in the supernatant is coprecipitated on BaC03. Separation from the Ba is effected by taking advantage of the insolubility of BaCl, in concentrated HC1-ether solution (72).


259

TABLE 6-Continued PADIOISOTOPE

EODUCED BY

DESCRIPTION OX SEPARATION PBOCESSES

The Sc203is dissolved in 12 N HCl and the solution is evaporated to incipient dryness. H2O is added and the pH is adjusted to 4.0 with NaOH. This solution is extracted with 200 ml. of 0.5 M thenoyl trifluoroacetone in benzene, which removes most of the Sc. The aqueous phase containing the Ca" is adjusted to pH 8 with NaOH and reextracted with 0.5 M thenoyl trifluoroacetone. The Ca46 in the benzene fraction is backextracted with H 2 0 or dilute HC1 (9). Ti (d,CY) Ti(d,Bp)

,948..

The T i metal target is dissolved in a minimum volume of H2S04.The solution is slowly added to excess 8 N NH4OH containing 30 per cent H 2 0 zto give a clear "solution" of the soluble pertitanate containing the Sc activity as radiocolloidal aggregates. This solution is then drawn through filter paper, which retains over 95 per cent of the Sc activity a~ adsorbed radiocolloid, which is removed by treatment with dilute HCl (39). The TiO, is fused with a mixture of Na2COaand NaNOa. The V48 is leached from the fused mass with water. The alkaline solution of V48is acidified with HCl and reduced in volume to precipitate the large excess of NaCl. V48 is retained in t h e supernatant (58). The V target is dissolved in "08. The solution is diluted t o 6 N and saturated with SO2 t o insure the reduction of C P a . Fe+++ or La+++is added and the solution is made alkaline with Na&Oa. V is oxidized t o soluble vanadate by air in alkaline solution and Cr51 is carried qualitatively on the La(OH)a. CrS1is separated from the La(OH)3by a second precipitation in the presence of Br2, which oxidizes the Crsl t o chromate (43). The Fe target is dissolved in HC1. All but a few milligrams of Fe is extracted with ether. The Mn62. b4 in the tetrapositive state is quantitatively coprecipitated on Fe(OH)a with the addition of NH40H-Br2 mixture. Several reprecipitations are required to obtain the Mn52, 64free from the concurrently produced Co activities (89). The Cr target is dissolved in HC1 and evaporated to incipient dryness. FeC13 carrier is added and the solution is added with stirring to NaOH solution saturated with Brz. The Mns*v 64 is quantitatively coprecipitated on the Fe(OH)3,which is dissolved in 6 N HCl and extracted with ether to remove Fe carrier ( 5 5 ) .

sFe69. . . . . . . , , .

The Co is dissolved in 3 N HNOa and the pH of the solution ia adjusted to 4.0-7.0 by the addition of NH4OH-NH4C2H3O2.

2GO

WARREN M. GARRISON A N D JOSEPH G. HAMILTON

TABLE 6-Continued EADIOlSOTOPB

RODUCED B'

DESCRIPTION OF SEPABATION PPOCPSSLS

1ml. of a saturated aqueous solution of acetylacetone is added and the solution is extracted with xylene. The organic phase containing the Fe5sis evaporated to dryness. Organic residue is destroyed by fuming HClO, (70). The Co target is dissolved in HCl. Excess NHIOH is added and the resultant clear "solution" is passed through two consecutive Whatman No. 50 filter papers, which quantitatively removed the Fe59 as adsorbed radiocolloid. The activity is removed from the filter paper with dilute HC1 (43). The Fe target is dissolved in HC1. Fe(OH)* is precipitated with the addition of excess NH4OH-NH4Cl solution containing Br2. The Go50 is retained in the supernatant. Ammonium salts are destroyed with HXOa (89). The Zn target is dissolved in HC1, diluted t o 5.5 N , and the Ga activities are extracted with ether to reduce radiation hazard in subsequent manipulations. The pH of the aqueous phase is adjusted to 1.0-1.2 with NaOH and the CuB4is extracted with CCl, solution containing 0.001 per cent dithizone. The CCl4 phase is evaporated to dryness and organic material is removed by heating to 500°C. (59, 113). ZnSO, target is dissolved in water. The Cu64is separated from the acidified solution by shaking the solution with a few milligrams of freshly precipitated Bi2S3,dissolving the BiISs evaporating the solution to dryness, taking up with "02, in dilute I-ICl, and reprecipitating the Bi as Bi(0H)a with excess "4OH. The Cue4is retained in the supernatant (4). ZnClz target is dissolved in water. CuS4 is concentrated by addition of a small amount of Zn, which is then dissolved in acid and Cue4 is deposited on polished Pt foil from 0.05 N HISO, saturated with HI (31).

:oZn'Ja.. . . . . . . .

Excess HCl is added to de3u(d, 2n) Cu target is dissolved in "Os. and the solution is neutralized to 0.25 N HC1 stroy "03 with ",OH. CuS is precipitated with H2S. Zn66 is not coprecipitated. The supernatant is adjusted to pH 5-5.5 with NaC2Ha0, and sufficient P\'a2S208is added to complex traces of Cu remaining in solution. The Znas is extracted into CC14 solution containing dithizone and back-extracted with 0.02 N HCl (120).

. .......

Zn(d,n) Zn target is dissolved in HC1 and adjusted to 6 N . Gas7 is exZn (d ,2n) tracted with diethyl ether (49).

arGe71. . . . . . . . . .

3a(d, 2n) The Ga target is dissolved in 48 per cent HBr and the solution is distilled. The Ge71 is obtained in the first few milliliters of

:lGa67.


261


'BODKJCED B


the HBr distillate. HNOa is added to destroy HBr and the solution is evaporated to incipient dryness. H I 0 is added and the evaporation is repeated t o remove traces of HNOa (82).

AS".

Ge(d, 2n

The Ge metal is dissolved by refluxing with aqua regia. Excess HCl is added to destroy HNO,, and GeCl, is distilled from the solution. The As74 is retained in the residue as the nonvolatile pentachloride. After GeC14 is completely removed, HBr is added to the HCI residue and As74, as the volatile trichloride, is distilled into "Os. The "0, solution of A874 is evaporated t o dryness (81, 83).

r4Se76.. . . . . . . . . . As ( d , 2n

The As is dissolved in aqua regia. HCI is added to destroy excess "03. The solution is adjusted to 3 N . Tellurous acid carrier is added and precipitated as Te by passing SO2 through the solution. Se76is carried quantitatively. The Te precipitate is dissolved in HN03. HBr is added and the T e is distilled. Se75is retained in the HNOI residue (37).

Se ( d ,2n

The Br8O, B2 is volatilized while the Se target is dissolved in H2S04.An inert gas is used t o sweep the Brao-82 into a t r a p filled with CClr. A major fraction of the activity is extractable with Na2SOa solution. Some of the Br80, 82 reacts with and does not extract the CC14 ( 3 6 ) .

Br ( d , 2n

The Kr7Qis retained in the KBr target material during bombardment. Separation is made by fusing the KBr or by boiling a solution of the KBr in uucuo (10, 52).

,&r'Q.

. . . . .. . . . . . . . . . . . ..

The RbCl is dissolved in 0.1 N HCl. FeCll carrier is added and precipitated with the addition of (NH4)2CO,-NH4OH jolution. The Fe(OH), containing the SrS6activity is dissolved in 6 N HC1 and the Fe is extracted with ether (98). 3ra9,8 0 is separated from fission-product mixture by complex :lution from organic zeolites; Sra9, and Ba140 are removed 3uccessively by 5 per cent citrate a t p H 5 (7, 127). (A more jetailed description of fission-product separation by ion2xchange methods is given in the section on rare earths.)

my=.

.. . . . . . . . . .

aeY 91 . . . . . . . . . . .

Sr(d, 2n)

The Sr metal is dissolved in dilute HCl. The solutionis diluted ;o -0.5 M Srf+ and NH4OH is added t o pH 9. This "solution" s then drawn through filter paper, which removes the YES is adsorbed radiocolloid. The activity is removed with dilute HCl (78).

Ysl is separated from rare-earth fission products by complex :lution from Amberlite IR-1resin with 5 per cent citrate at 2H 2.7 (71, 127). (A more detailed description of fissionxoduct separations is given in the section on rare earths.)

TABLE &-Continued BADIOlSOTOPE

DESCRIPTION OF SEPABATION PBOCBSSES

roZr*Q.. . . . . . . . .

The YzO3 target is dissolved in HC1. The resultant solution is evaporated almost to dryness and then diluted with HzO. Y is precipitated as the fluoride with HF. A few milliliters of H&04are added and H F is removed by evaporation. The solution is diluted with water. FeC13 carrier is added and precipitated with NH40H. The Zr89 is carried quantitatively. The precipitate is dissolved in 6 N HC1 and Fe is extracted with ether (98).

toZr06... . . . .

Oxalic acid is used as a specific complex-forming agent t o remove ZrQ6and CbQsfrom mixed fission products on Amberlite IR-1 columns. 0.5 per cent oxalic acid strips Zr@6-Cbs6quantitatively; none of the trivalent or divalent elements is removed by this procedure (127). Zrg6is separated from Cbes daughter by ( a ) coprecipitation of Cbs6 on MnOz from 10 N HNOa and ( b ) extraction of Zrg6 from HNOS or HClO4 using thenoyl trifluoroacetone (18). The bulk of the uranyl nitrate target material is removed by ether extraction. To the concentrated solution of fission products are added a few milligrams of FeC16,hydroxylamine, and excess “,OH. Zrg6and other insoluble fission products, in the presence of NHIOH, are coprecipitated on the Fe(OHh. The U remains in the supernatant. The Fe(OH), is dissolved in HC1 and Fe is extracted with ether. The Zr96, Cbg6,and Ce141, 144 are separated as iodates using thorium carrier. Zr9hCbs6 are separated from Ce and T h by means of H F precipitation. Cb96 is separated from the Zr96 by K2COs fusion (98).

41Cbg6.. , , . . . . . .

See ZrQ6procedures.

4&fOgg. .. .. . ., ..

The ZrOz target is dissolved in the presence of Cb “holdback” carrier by boiling in 12 N HZS04 down t o fuming. NaCl and HC1 are added, following cooling and dilution with water, t o bring the (H+) and (Cl-) t o 6 N . The Mo is extracted with ether saturated with HC1 (121).

4tTcQ‘. . . . . . . . . .

The Mo target is dissolved in aqua regia and evaporated t o incipient dryness. The oxide is dissolved in HClO4-HlPO4 and Tcg6 is distilled with the addition of HBr, using a COZ carrier gas ( 6 , 90).

--

4rTc99 . . . , . . . . . .

The U metal is dissolved in 37 per cent HC1 and the UC1 is oxidized t o UOzClz with Hz02 and Brz a t 60°C. PtCl4 is added and the solution is saturated with HIS at 90°C. The Tcg9is coprecipitated with the PtSp along with other acid-insoluble sulfides of the fission products. The PtSp is dissolved in H2Or “,OH solution, BrPis added, and the solution is evaporated t o incipient dryness. 18 N H2SOc is added and the Tc99 is distilled (101). 262

TABLE 6-Continued PISlOlSOIOPE

PRODUCED BY

PESCBIPTION OF SEPABAIION PROCESSES ~

~

44R~'~s IO3. . . . . . . MO(CY, The Mo target is fused with KOH and KNO,. The resultant melt is extracted with water. The basic solution is transzn) ferred t o a distilling flask and the carrier-free Rug?,1 0 3 is volatilized by passing Clz through the solution with heating. The Ru activity is collected in a trap filled with 12 N HC1. The distillate is evaporated t o a small volume, treated with excess KOH, and the Rug's l o 3 is redistilled as above (43). The U metal is dissolved in H N 0 3 and adjusted t o 3 N . N a I is added and the solution is boiled until all of the iodine is expelled. HC104is added and Ru103is distilled into 3 N HC1. The distillate is made strongly alkaline and Rulo3is distilled from alkaline solution after oxidation with Clz (98). The Ru target is fused with Na20z and the melt is digested with HCl. After centrifugation, the acid solution is made basic with KOH and the Ru is volatilized by passing Clz through the solution with heating. The residue is acidified, 5 mg. of Fe+++is added, and the RhlO1*1 0 2 is coprecipitated on Fe(OH)3 with the addition of KOH. The Fe(OH), is dissolved in 6 N HC1 and Fe is separated by ether extraction (43). 4

Jd'o' a.

. . . . . . . , . Rh(d, 2n)

The Rh is fused with K H S 0 4 and the fused mass is dissolved in water. HCl is added together with milligram amounts of HzSe04and the solution is saturated with SOZ. The Pdloais carried quantitatively on the Se metal. Se is removed by distillation with HC104 (44). Ag106is separated by coprecipitation on HgZC12, which is removed by volatilization (57). Aglll is separated from Pd by electrolysis from 1 M NaOH0.1 M NaCN solution using Pt electrodes and a cathode potential of - 1.210 v. (50). ~

~~~

The Ag is dissolved in "0% and the solution is evaporated t o dryness. The AgN03 containing the activity is dissolved in HzO, and the Ag+ is combined in a complex with NH,CNS. The solution is adjusted t o p H 5 with sodium acetate, and the Cd109is extracted with chloroform containing 5 per cent pyridine (92). After separation of Sn1I3by distillation (see below), the HzSO4 residue containing In114 is neutralized with ",OH. The Fe(OH), precipitate carries in the 1x1"' quantitatively in the presence of the N H 4 0 H . Fe is separated by ether extraction (91). The Cd is dissolved in HN03 and the solution is evaporated t o incipient dryness, diluted with HzO, and made alkaline with ",OH after the addition of FeC13. The Sn"3 and 263

264

WARREN M. GARRISON A N D JOSEPH G. HAMILTON


'PODUCED BY


are carried on the Fe(OH)3 precipitate. The Fe(OH)3 is dissolved in a minimum volume of 36 N H&O4 and transferred to an all-glass distilling apparatus. HBr is added dropwise and Sn"3 is volatilized at 220°C. The distillate is collected in HC1 (91). The Sn target is dissolved in aqua regia. HCl is added to destroy excess HNO, and the solution is adjusted to 0.1 N HCl. Milligram amounts of Cd++ are added and precipitated with HzS after the addition of oxalic acid t o prevent the precipitation of SnSz. The CdS is dissolved in HCl and transferred t o a divtilling flask. Traces of Sn are removed by distillation with HC10, a t 200°C. Sb1200 122 is then distilled with the gradual addition of HBr (91). The Te1270 129 is precipitated out of an HCl fission mixture on CuS. The CuS is dissolved in HNOa and the solution is made 5 N in HF. The Tela78 1 9 9 is then coprecipitated on nus,. The RuSl is decomposed with HNOa and the residue is boiled with HC104 to expel the Ru (98). 6 p 1 .

.... . . . . ..

The Te is dissolved in CrOa-HzS04solution. Oxalic acid is added and the I* is distilled into XaOH (102). 1131 vaporizes during the solution of U in HNO1. The vapor is passed through a condenser and NaOH scrubber. Ilalis redistilled into dilute NaOH from HNO3-H~Oz. The NaOH solution of 1131 is made 20 per cent in H2SO4 and treated with KMnOa. Oxides of nitrogen are removed by distillation. The residue is treated with HJPOa and the 1"' is distilled into NaOH-NanSOa solution (1, 110).

The T e metal is fused with NaOH. The fused mass is extracted with water and the washings are transferred to an all-glass disctilling flask. KMn04 is added to the alkaline solution t o oxidize 1131 to IO3-. HpS04 is then added, followed by solid oxalic acid and the 1131 is distilled into dilute Na2SOa-Na&Oa solution (84). Te target is dissolved in 18 N HZSOr50 per cent CrOa solution. The solution is cooled to 50°C. and solid oxalic acid is slowly added. The P1is then distilled into dilute NanSOr NaOH solution (1, 110). 6,Xe1t7.. . . . . . . .

The Xell7 is retained in the K I target material during bombardment. Separation is made by fusing the KI or boiling 5 solution of the K I i n vacuo (52, 98).

* I is evolved during the dissolution of Te target in "08. a trap filled with CCl, and extracted with NazSOI (98).

The activity is collected in

TABLE 6-Continz~ecl ~~~

I

RADIOISOTOPE

IPRODUCED BJ


BaC12 is precipitated from 12 N HCl saturated with Clz a t Ba131 @--decay 0°C. Cs'31 remains in the supernatant. Further purification is obtained by scavenging with La(OH)3 (34).

U(n,f)

6scs137.

Cs137 is separated from the divalent or trivalent fission product ions by preferential elution from Amberlite IR-1, using 0.25 hr HC1 (100). Cs137 is separated from fission mixture by coprecipitation on NH4C10, from HClO, solutions using absolute alcohol (45). Cs137 is separated from a fission-product mixture by coprecipitation on sodium cobaltinitrite and ammonium cobaltinitrite. The Co is removed as COSfrom NHdOH solution (98).

,Balas. . . . . . . . . . Cs(d ,2n: The CsCl target material is dissolved in HC1 to pH 2 and the solution is adjusted t o pH 10 with K H 4 0 H after the addition of FeC13 carrier. The precipitate of Fe(OH)3 is washed, dissolved in HCl, and reprecipitated with ",OH a t pH 5.4. The Bala3is retained in the supernatant (79, 98). After the elution of the rare earths from Amberlite IR-1, using 5 per cent citrate at. pH 2.7, the Sr*9, 9 0 and Bal4O are successively removed by 5 per cent citrate at pH 5 (112, 127).

... . . . .. .

s7~,a14~.

La140 is separated from Bal4O-Lald0mixtures by elution from Amberlite IR-1, using 5 per cent citrate a t pH 2.7 (112, 127). BaC12 is added t o an IIC1-Ba140 solution and precipitated by passing HC1 gas through the solution a t 0°C. The BaC12 containing the BaI40 is dissolved in HzO. FeCI3 carrier is added and precipitated with the addition of ",OH. The Fe(OH)* precipitate is discarded. After allowing the purified BaClz to stand for several days a second Fe(OH)3 precipitation is made which carries the La140.Fe is removed by ether extraction from 6 N HC1 (98).

57-63. . . . . , . . . LaI4O Prl'a Nd"7 Pm*47 Sm"1 Eu155

.

,

A solution of all fission-produced species in 0.01-0.1 N HC1 is passed through an Amberlite IR-1 resin column. All cations are adsorbed while the fission-produced anionic elements (Ru, Te, Tc) and any I present which pass through the acid are recovered in the effluent. hlost of the Cs13' may be eluted a t this point by the addition of 0.25 N HCl. The column is washed, and Cb95-Zrg5 is preferentially removed by 0.5 per cent oxalic acid. By using differential pH-controlled ammonium citrate elution, the trivalent elements are removed i n the order: Y, E u , Sm, Pm, Nd, Pr, Ce, La. With Amberlite IR-1, the eluting agent in general use is 4.75-5.0 per cent citric acid a t a pH of from 2.7 to 2.95. I n practice a group of several elements may be eluted, then readsorbed and fractionated under more- rigidly controlled conditions (100, 110, 127). 265

266

WARREN M. GARRISON AND JOSEPH 0. HAMILTON

TABLE &Continued RADIOISOTOPE

64-7 1 . . . . . . . . , , Gd Tb

DY Ho Er Tm Yb

Lu

'RODUCED BY


After bombardment, the rare earth oxide is dissolved in "03 and insoluble matter is removed by centrifugation. Carriers for contaminating activities are added and the rare earth fluoride is precipitated from hot 2 N HNOa by addition of HF. The fluoride is dissolved in HN03-H3B03 and precipitated as the hydroxide. The rare earth hydroxide is dissolved in HC1 and the solution is adjusted to a pH between 0.5 and 1.5. The rare earth activities are then adsorbed onto Dowex-50 and eluted with 0.25 M citric acid adjusted to pH 3.05 with NHaOH (93, 132). ~~~~~

~

~

The HfO2 is dissolved in "0,-HF solution. HF is removed by evaporation with the addition of 16 N HN03, Khln04 is added, and the MnOz precipitate carries the T a quantitatively. The MnOz containing the T a activity is dissolved in oxalic acid and Mn* is removed by adsorption on Amberlite IR-1 or Dowex-50 (56). The T a metal target is dissolved in a minimum volume of 16 N HN03 containing 10 per cent HF. The H F is removed by volatilization and the bulk of the tantalic acid is separated by centrifugation. The Re183* is quantitatively retained in the supernatant. The HN03 solution is evaporated almost to dryness and transferred to an all-glass distilling flask with 36 N HzS04.The Re183. 184 is distilled at 240°C. with the addition of 9 N HBr and coprecipitated on CuS after the removal of HBr and HN03. Separation from the Cu is effected by precipitating CuO from alkaline solution (41, 43). The W target is fused with KOH-KN03 and the fused mass is extracted with water. The solution is acidified with 16 N HNOa,precipitating tungstic acid which is removed by centrifugation. The carrier-free Re183t 184 is isolated from the HNO, supernatant, using the distillation procedure described above (42). rsOs1'6. . . . . . . . .

The W metal is fused with KOH-KNOa and the fused mass is extracted with water. The solution is acidified with HNOa, precipitating most of the wo3, which is removed bycentrifugation. The Os186 is distilled from 5 N HNOa and collected in a trap filled with 5 N HN03. Extraction of the "01 distillate with ether removes the 0 ~ 1 8 6 ,which is back-extracted with NaOH (42).

Os(d,xn) The Os powder is dissolved in aqua regia and volatilized with the addition of excess "03. The residue is fumed with t o remove Re activity and then heated to dryness. The carrierfree Ir1903 1 9 2 is redissolved in 12 N HC1 and the resultant solution is evaporated to dryness on NaCl (50).


267

TABLE 6-Concluded RADIOISOTOPE

PODUCED B Y


T1(d,2%) The T1203target material is dissolved in dilute HN03. Tlis reduced t o TI+ with SO2. Five milligrams of Few is added and the solution is made basic with NHdOH. The Pb20’ is coprecipitated with the Fe(0H)o and the Fe removed by ether extraction (56). Pb (d,2n) The P b is dissolved in a minimum volume of HNOl and the resultant solution is evaporated to dryness. The Pb(NO,)r containing the Bi204.2 0 6 is dissolved in excess 10 per cent NaOH. This “solution” is drawn through filter paper which retains over 98 per cent of the Bi activity as adsorbed radiocolloid. Dilute HC1 removes the Bizo4f z o 6 quantitatively (40). The P b target is dissolved in “ 0 3 . Pb(NO& is centrifuged off and the solution is extracted with amyl acetate to remove T1, Hg, and Au. Bi and T1 hold-back carriers are added and the solution is fumed with HCl. The solution is diluted t o 3 N and Te carrier is precipitated with SO2, removing At and Po. The T e is reprecipitated from 6 N HC1. Under these conditions A t is coprecipitated and Po remain6 in the supernatant. Po in the HC1 solution is extracted with an equal volume of 20 per cent tributyl phosphate in diethyl ether. The activity is back-extracted with HNO, (93).

A layer of Bi metal alloyed to a 10-mil. thick water-cooled A1 target plate is bombarded in a bell-jar target filled with He lor additional cooling and to prevent oxidation of the Bi. The bombarded Bi is cut from the target foil and At is isolated by heating the Bi t o 425°C. in a stream of NOcarrier-gas. The At is collected on a cold-finger cooled with liquid air (35, 67).

2. Radiochemical contamination

Although many of the possible sources of radiochemical contamination in radioisotope preparations can be eliminated by a proper choice of target materials, bombardment energies, separation process, etc., radiochemical analysis of the final preparation is usually required. I n determining radiochemical purity, the safest procedure is to employ both physical and chemical methods, the former based on consideration of half-life and radiation characteristics and the latter involving conventional analytical separations using added stable isotopic carriers for each of the radioelements known to be produced or suspected of being produced under the particular bombardment conditions employed. Detailed procedures for the detection and identification of radioactive contaminants have been discussed in considerable detail elsewhere (17). C. TARGET PREPARATIONS

From the standpoint of yield of radioactive product per gram of bombarded material, the most desirable target substance for both cyclotron and pile bom-

268

W A R R E N M. GARRISON A N D JOSEPH 0. HAMILTON

bardment is a single isotope in the elemental form. This situation is infrequently realized, of course, because most elements are polyisotopic and the use of separated isotopes is usually not warranted in the practical production of carrierfree tracers. I n addition, factors other than those based on yield considerations may be of paramount importance in determining the physical and chemical composition of target materials. Probably the most important factor limiting the choice of cyclotron target material is the problem of dissipating the large heat input from the bombarding beam of particles. With an “internal” or “probe” target assembly, which is supported inside the vacuum chamber of the cyclotron, beam currents as high as 1 milliamp. may be obtained over an area of approximately 0.5 cm.2 requiring a power dissipation of -40 kw./cm.2 for a 20-M.e.v. deuteron beam. Temperature gradients in the order of 200O0C./mm. may be produced under these conditions. Target materials for probe bombardments are limited to pure metals or metallic alloys having a high melting point and a high heat conductivity. Even for these substances the power dissipation cannot usually be greater than approximately 10-20 kw./cm.2 The metals are generally soldered or plated to a water-cooled support which may be made to rotate or vibrate, thus spreading the incident beam over a larger area (87). With the “bell-jar” target assembly, a fraction of the internal beam is brought out of the cyclotron vacuum through a thin metal “window” into an external enclosure which can be evacuated or filled with any desired gas. The intensity of the emergent beam is usually about one-tenth of the internal current. Maximum available external-beam intensities are rarely used except in the bombardment of very refractory materials or in the bombardment of metals and metallic alloys which have a high thermal conductivity and can be bonded to a watercooled target plate. Additional cooling can be obtained by passing an inert gas through the target chamber. Target materials which volatilize or decompose t o give a gaseous product must be irradiated with the external beam. Metal powders, oxides, and other substances which are not readily bonded to the target plate are supported with very thin metal foils, usually of platinum or tantalum. Many different target designs have been developed for special uses (80, 104, 129, 133). The authors wish to thank Professor G. T. Seaborg, Professor R. Overstreet, and Professor L. Jacobson for valuable advice, criticism, and suggestions, Mr. J. R. Mason and Mr. A. F. Rupp of the Oak Ridge National Laboratory for making available to them a series of declassified reports on separation methods for several of the fission-produced radioisotopes, Mr. Herman R. Haymond and Miss Jeanne D. Gile for many helpful discussions and assistance in the literature survey, and the staff of the 60-in. cyclotron a t Crocker Laboratory for much of the information on target design, preparation, and bombardments. They are also grateful to Mrs. Betty Stallings for her painstaking work in the preparation of the manuscript and Mrs. Alberta Mozley, Mrs. Helen Haydon, and Miss Margaret Gee for technical assistance.


269

IV. REFERENCE$ BALLANTINE, D . S., A N D COHN,W. E.: MDDC-1600 (June, 1947). BALOMEY, R. A., A N D WISH, L.: AECD-2665 (August, 1949). BARSCHALL, H . H., A N D BATTAT,M. E.: Phys. Rev. 70, 245 (1946). BORN,H. J., A N D DREHMAN, U.: Naturwissenschaften 32, 159 (1945). BOUCHEZ, R., A N D KAYAS,G.: Compt. rend. 228, 1222 (1949). BOYD,G. E., LARSON, 0. V., A N D MOLTA,M. E . : AECD-2151 (June, 1948). BOYD,G. E., SCHUBERT, J., A N D ADAMSON,A. W.: MDDC-708 (1947). BRADT,H. L., A N D TENDAM, D . J.: Phys. Rev. 72, 1117 (1947). BROIDO,A.: AECD-2616 (June, 1949). BROWN,S. C., IRVINE, J. W., AND LIVINGSTON, M. S.: J. Chem. Phys. 12, 132 (1944). (11) BUCK,J. H.: Phys. Rev. 64, 1025 (1938). (12) CAILLAT, R., A N D SUE,P.: Bull. soc. chim. France 1949, 687. (13) CLARKE, E. T.: Phys. Rev. 71, 187 (1947). (14) CLARKE, E . T., A N D IRVINE, J. W., JR.:Phys. Rev. 66,231 (1944). (15) CLARKE, E. T., A N D IRVINE, J. W., J R . : Phys. Rev. 69, 680 (1946). (16) COHN,W. E . : Isotope Branch Circular No. C-1, U. S. Atomic Energy Commission (1947). (17) COHN,W. E.: Anal. Chem. 20, 498 (1948). (18) CONNICK, R. E., A N D MCVEY,W. H . : J. Am. Chem. SOC.71, 3182 (1949). (19) CORK,J. M.: Phys. Rev. 70, 563 (1946). (20) CORK,J. M., A N D CURTIS,R. B.: Phys. Rev. 66, 1264 (1939). (21) CORK,J. M., A K D HALPERN, J.: Phys. Rev. 67, 667 (1940). (22) CORK,J. M., HALPERN, J., AXD TATEL, H.: Phys. Rev. 67, 371 (1940). (23) COVEY, E . H.: UCRL-266 (March, 1949). (24) CROWLEY, J., HAMILTON, J. G., A N D SCOTT,K.: J. Biol. Chem. 177, 975 (1949). H. H., LINDER,M., MILLER,D . R., O'CONNOR, (25) CUNXINGHAM, B. B., HOPKINS, P. R., PERLMAN, I., SEABORG, G. T.,A N D THOMPSON, R. c.: Phys. Rev. 72, 739 (1947). (26) DELSASSO, L. A., RIDENOUR, L. N., SHERR,R., A N D WHITE,M. G.: Phys. Rev. 66, 113 (1939). (27) DREHMANN, V. : Naturwissenschaften 33, 24 (1946). (28) D u BRIDGE,L. A., BARNES,S. W., BUCK,J. H., AND STRAIN,C. V.: Phys. Rev. 63, 447 (1938). (29) EDWARDS, R. R., AND DAVIES,T. H.: Nucleonics 2, 44 (1948). (30) ERBACHER, 0.: Z. physik. Chem. B42, 173 (1939). (31) ERBACHER, O., HERR,W., A N D EGIDI,V.: Z. anorg. Chem. 266, 41 (1948). O., A N D PHILIPP, K.: Ber. 69, 893 (1936). (32) ERBACHER, (33) FINKLE,B., A N D CORN,W. E.: AECD-2556-F (March, 1949). K . , A N D VOIGT,A. F.: Phys. Rev. 60, 619 (1941). (34) FAJANS, (35) GARRISON, W. M., GILE,J. D., MAXWELL, R. D., AND HAMILTON, J. G.: UCRL-741 (May, 1950). (36) GARRISON, W. M., A N D HAMILTON, J. G.: Unpublished data. (37) GARRISON, W. M., MAXWELL, R. D., A N D HAMILTON, J. G.: J. Chem. Phys. 18, 155 (1950). (38) GHOSHAL, S. N.: Phys. Rev. 73, 417 (1948). (39) GILE,J. D . , GARRISON, W. M., A N D HAMILTON, J. G.: UCRL-921 (September, 1950). (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)

Those references designated by the symbols AECD and MDDC refer t o declassified U. S. Atomic Energy Commission publications. The symbol UCRL refers t o a University of California Radiation Laboratory publication.

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GARRISON A N D JOSEPH GI. HAMILTON

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